There is a rising demand to increase fuel economy in a wide range of vehicles, including passenger vehicles, commercial vehicles, such as tractor trailers, and off-highway vehicles, such as mining and construction equipment. One of the ways to increase fuel economy is to reduce the size of the engine for any gasoline powered vehicle. Of course, if the engine size is reduced, available power is also reduced unless a supplemental power source for high or increased power demands can be selectively engaged.
Selective engagement opportunities might be during high power demands, such as when the vehicle is going up a grade, passing, starting or other working conditions. A supplemental power source permits an internal combustion engine to be reduced in size so that it can still handle a wide range of power needs of the vehicle, but the internal combustion engine need not be sized to meet every possible need. Instead, the supplemental power source may be used to selectively add power to the vehicle at high demand times. It may also be permissible or desirable for the supplemental power source to recover energy from the vehicle and then use that recovered energy to power the source as well as the vehicle.
One possible supplemental power source for vehicles may be such as a mechanical flywheel. Flywheel energy storage systems work by accelerating a rotor or disc to very high speeds via an external device, such as an internal combustion engine, an electromagnet, or an axle. The available kinetic energy in the system can be transferred into rotational mechanical energy, thus providing a power source to the driveline. The rotating flywheel can also be used as a power sink during braking. When energy is extracted from the flywheel, a rotational speed of the flywheel is reduced as a consequence of the principle of conservation of energy; adding energy to the flywheel correspondingly results in an increase in the speed of the flywheel.
In one example, a flywheel energy storage system can be connected to the front or rear axle of a vehicle. During periods of deceleration, braking energy is used to speed up the flywheel (up to about 60,000 revolutions per minute, for example). When the vehicle accelerates, the rotational energy from the flywheel is transferred to mechanical energy to the driving wheels of the vehicle via a specially designed device, like a continuously variable transmission, for example.
A known driveline layout for a vehicle driveline 100 equipped with a flywheel 102 is depicted in FIG. 1. As shown in the figure, a power source 104 (such as an internal combustion engine or an electric motor, for example) is connected to a clutch 106, which is connected to a transmission 108, which is connected to an axle 110 and a pair of wheels 112. The flywheel 102 is schematically depicted as being connected to an output 114 of the power source 104. The vehicle driveline 100 layout has several disadvantages that must be overcome.
A first disadvantage of the driveline layout shown in FIG. 1 is synchronizing a varying speed of the flywheel 102 with a varying speed of a vehicle (not shown) the vehicle driveline 100 is incorporated in. The varying speed of the flywheel 102 is dependent on an amount of energy stored therein. Accordingly, if a portion of the amount of energy stored in the flywheel is transferred to the vehicle driveline 100, a speed of the flywheel 102 drops. Each of the speeds in the vehicle driveline (a speed of the power source 104, a speed of an input 116 of the transmission 108, a speed of an output 118 of the transmission 108, for example) is related to a road speed of the vehicle. As a non-limiting example, the power source 104 may have a rotational speed that varies between about 1000 revolutions per minute and about 3000 revolutions per minute; resulting in a spread factor of about 3. The flywheel 102 may have a rotational speed that varies between about 30,000 revolutions per minute to about 60,000 revolutions per minute; resulting in a spread factor of about 2. The rotational speed of the power source 104 is linked to the road speed of the vehicle, and the flywheel 102 must be able to be drivingly engaged therewith. Therefore, a device capable of providing a total spread factor of about 6 (a spread factor of about 2 multiplied by a spread factor of about 3) would be required to drivingly engage the flywheel 102 with the vehicle driveline 100.
A second disadvantage is a difficulty in smoothly connecting the vehicle driveline 100 with the flywheel 102. If the flywheel 102 was infinitely rigidly connected to the driveline with an appropriate ratio, at a later point the ratio between the flywheel 102 and the road speed of the vehicle would not be valid anymore and the flywheel 102 would provide either too much torque or not enough torque. Further, pairing the vehicle driveline 100 and the flywheel 102 influences the rotational speed of the flywheel 102 and thus the amount of torque provided by the flywheel 102. Accordingly, for the vehicle driveline 100 to be capable of engaging the flywheel 102, the vehicle driveline 100 must permit small errors in the ratio set to occur.
It would be advantageous to develop a driveline and a method for transferring energy from a flywheel that increases a fuel efficiency of a vehicle the driveline is incorporated in, permits a primary power source to be selectively supplemented using the flywheel, and permits the flywheel to store and capture excess energy present in the driveline.